The present invention relates to a catalyst and process for the preparation of hydrocarbons from synthesis gas, that is a mixture of carbon monoxide and hydrogen.
The preparation of hydrocarbons from synthesis gas is well known in the art and is commonly referred to as Fischer-Tropsch synthesis.
Catalysts suitable for use in a Fischer-Tropsch synthesis process typically contain a catalytically active metal of Group VIII of the Periodic Table of the Elements (Handbook of Chemistry and Physics, 68th edition, CRC Press, 1987-1988). In particular, iron, nickel, cobalt and ruthenium are well known catalytically active metals for such catalyst. Cobalt has been found to be most suitable for catalyzing a process in which synthesis gas is converted to primarily paraffinic hydrocarbons containing 5 or more carbon atoms. In other words, the C.sub.5+ selectivity of the catalyst is high.
Much research effort has been directed to finding catalysts having a higher C.sub.5+ selectivity than known catalysts at the same or higher activity.
Thus, European patent specification No. 398 420 describes that the C.sub.5+ selectivity of catalysts comprising cobalt and zirconium, titanium or chromium on a porous carrier, having a small external surface area, can be improved by contacting the catalyst with a synthesis gas having a low hydrogen to carbon monoxide ratio, typically, from 1.1 to 1.2.
European patent specification No. 178 008 discloses cobalt catalysts supported on a porous carrier, wherein most cobalt is concentrated in the peel of the catalyst particle.
European patent specification No. 167 215 discloses a cobalt/zirconia on silica catalyst for use in a fixed catalyst bed which catalyst satisfies a relation between the internal surface area and the external surface area.
European patent specification No. 168 894 discloses an optimal activation procedure to increase the C.sub.5+ selectivity of a cobalt-based Fischer-Tropsch catalyst.
European patent specification No. 363 537 describes an increase in activity of cobalt catalysts supported on titania, by adding up to 15% by weight of silica to the titania carrier.
European patent application publication No. 498 976 describes catalysts containing cobalt and rhenium supported on a titania carrier. It is claimed that these catalysts have a high volumetric productivity (activity).
Despite the research effort in this field there is still room for improvement. Accordingly, it would be desirable if a catalyst could be found which has an even higher C.sub.5+ selectivity at the same or, preferably, higher activity than known catalysts. In particular, it would be desirable if a catalyst could be found having a high C.sub.5+ selectivity and activity when contacted with a synthesis gas feed having a high hydrogen to carbon monoxide ratio, typically in the range from 1.5 to 2.5.
It has now surprisingly been found that a catalyst comprising as catalytically active compounds cobalt and a small amount of manganese and/or vanadium, typically comprising a cobalt:(manganese+vanadium) atomic ratio of at least 12:1, exhibits a higher C.sub.5+ selectivity and a higher activity when used in a process for the preparation of hydrocarbons, compared to catalysts which are otherwise the same but containing cobalt only, or containing a relatively higher amount of manganese and/or vanadium.